Players of Zynga’s online games will be able to earn rewards through a prepaid card using American Express’ Serve digital payments platform under a deal announced yesterday between American Express and Zynga.

The Zynga Serve Rewards program is initially available to players of “FarmVille,” a popular Facebook-based game from Zynga. Similar programs will be introduced in additional games such as “CastleVillle” and “CityVille” over the course of the year, so that players can redeem rewards to buy virtual goods from the games.

Under the collaboration, “FarmVille” players can plant an interactive “Serve Money Tree” in the virtual farm they create through the game. Planting a tree will enable players to earn Zynga Farm Cash that can be redeemed for virtual rewards.